The beginning of the war in 1939 changed the geopolitical situation in the Baltic region. After Poland had lost its sovereignty and the Soviet Union approached the borders of Lithuania, the country’s defence concept also changed. Reforms of the army that had started in Lithuania as early as 1935 fundamentally changed the approach to the defence of the country. The vision was declared that every citizen was a defender of his country. After the reinforcement of border control, mobilisation plans were drawn up, and the Riflemen’s Union was included in Lithuania’s defence plans. In 1939, after the Mutual Assistance Treaty had been signed with the Soviet Union, 20,000 Soviet soldiers entered Lithuania. Given the new geopolitical circumstances, a detailed restructuring of the concept of national defence was undertaken. This paper looks into the directives for mobilisation drawn up in the spring of 1940, the mobilisation plan for the Riflemen’s Union, and the plans for the use of the riflemen for national defence.